Mechanically actuated fuel injector units have been in use for many years. Continually increasing demands for improvements in vehicle performance and fuel economy, however, have escalated the need for more sophisticated fuel injection systems. Microprocessor technology has become not only a cost-effective means for meeting the demands of the present but appears to have the potential for meeting those of the future.
Associated with the application of microprocessor technology has been the development of electronically actuated fuel injectors. The development coincides with the steady increase in the total drive train reliability provided by the industry to reduce maintenance cost and regular maintenance frequency. Electronically controlled fuel injectors have the advantage of being compatible with the electronically controlled engines used in the general transport industry and have been adopted by major producers of engines.
A typical mechanically actuated fuel injector has a plunger that is reciprocatingly driven within a bore, or bushing, by, for example, a camshaft and rocker arm assembly, to provide injection pressure. Injection timing and fuel metering are controlled by helices and ports disposed in the plunger and associated bushing.
In a typical electronically actuated fuel injector, such as shown in U.S. Pat. No. 4,568,021, assigned to the assignee of the present invention, injection pressure is provided by a mechanically operated plunger; but a solenoid is used to actuate a valve to control injection timing and fuel metering.
It is as a result of the transfer of control of the timing and metering from mechanical to electronic means that improvements in fuel injection system operation under microprocessor control have been feasible. Included among additional advantages of electronically controlled fuel injectors are fewer moving parts, less weight, less maintenance as a result of there being fewer service adjustments required to compensate for mechanical wear, and less cost.
However, one design area requiring special attention is that of assuring the integrity of the solenoid stator assembly from any deleterious effects of it being exposed to the fuel, which is under exceedingly high pressures, in the order of 2,000 pounds per square inch. Each interface of the stator core with the phenolic housing and phenolically enshrouded coil on the center pole piece is subjected to fuel under high pressure which will work to separate the assembly at the interface, which may lead to hairline fractures in the phenolic housing and require its replacement. Applicants' initial commercially practical design modifications included providing the outer side of each outer pole piece with a T-shaped groove such that, when the phenolic housing was molded about the stator and coil subassembly, the housing was mechanically interlocked with the stator. This improved the overall durability of the assembly; but over time the high pressure fuel, primarily at the remaining pole piece interfaces with the phenolic insulating material, continued to adversely effect durability.
In part, the problem associated with the accessibility of high pressure fuel to these interfaces was exacerbated by the process with which the phenolic insulating material was molded about the stator and coil subassembly. This process included locating the stator and coil subassembly within the mold by means of vertically extending locating pins received within locating holes formed within a phenolic washer positioned between the pole pieces at the distal ends thereof. The locating holes provided a flow path by which the high pressure fuel gained access to the interior interfaces of the pole pieces, which over time could work a separation at these interfaces.
Thus, with the known solenoid stator assemblies, the insulating cover material, which relies solely on the strength of the bond between it and the stator core, may become separated from the stator core and show hairline fractures as a result of the fuel being forced between the stator core and the cover material, due to portions of the stator core to which the cover material is bonded being flexed, and due to cavitation erosion associated with fluid dynamics between a reciprocating armature and the stator core.
In part also, the problem associated with the accessibility of high pressure fuel to these interfaces and the propagation of hairline fractures was exacerbated by the material characteristics of the phenolic used for the housing and coil spool, which were found to be susceptible to swelling when exposed to methanol fuel especially, and to a lesser extent, diesel fuel.